FIG. 1 depicts a conventional Gallium Nitride (GaN)-based semiconductor device 100 fabricated on an insulating sapphire substrate 114. This device can be for applications such as a Light Emitting Diode (LED), Laser Diode (LD), Hetero-junction Bipolar Transistor (HBT) and High Electron Mobility Transistor (HEMT). During the conventional process, the device is formed on a sapphire substrate and both electrical contacts are formed on the top side of the device. A p-contact 102 is formed on the top and mesa etching is employed to remove material to form an n-metal contact 116. The result is called a lateral structure device and tends to exhibit several problems including weak resistance to electrostatic discharge (ESD) and heat dissipation. Both of these problems limit the device yield and useful life time. In addition, the sapphire material is very hard, which creates difficulty in wafer grinding and polishing, and device separation. Device fabrication yield is dependent on post fabrication processes including lapping, polishing, and die separation.
FIG. 2 depicts a second conventional technique that has become useful in building vertical structure GaN-based compound semiconductors 200. A laser lift-off (LLO) process is used to remove the sapphire substrate from the GaN epitaxial layer by applying an excimer laser having wavelength transparent to sapphire, typically in the UV range. The devices are then fabricated by substituting the insulating sapphire substrate with a conductive or semi-conductive second substrate 218 to build vertical structure devices. These processes typically employ wafer-bonding techniques for permanent bonding to the second substrate after removing sapphire substrate by laser lift-off.
However, there is still lacking a large scale laser lift-off process for the mass production of VLEDs (Vertical LED). One reason is the difficulty in large area laser lift-off due to non-uniformity of bonding adhesive layer 216 between support wafer 218 and the epitaxial layer 214 and the permanent second substrate 218 since the epitaxial layer surface is not flat over entire wafer surface after laser lift-off. Another problem associated with this wafer bonding technique is the degradation of metal contacts due to high temperature and high pressure during eutectic metal bonding process. Furthermore, substrates such as Si or GaAs used for the permanent wafer bonding are not optimal substrates in terms of heat dissipation compared to a Cu-based metal substrate. These problems reduce the final yield and do not provide a satisfactory solution to mass production of commercially viable devices.
FIG. 3 depicts a structure 300 intended to overcome the wafer bonding problems and fabricate VLEDs. Instead of using a wafer bonding method, the fabrication of device 300 includes attaching a metal support 318 to the device. However, the yield is known to be low due to de-lamination of the bonding layer during the laser lift-off process. If the bonding is not secure against the high-energy laser shock wave, the GaN epitaxial layers may buckle or crack after laser lift-off, and then it is difficult to perform post laser lift-off processes, such as wafer cleaning, device fabrication, de-bonding and device separation. Consequently, final device process yield is low.
Another problem of a vertical devices based on the technique shown in FIG. 3 is poor device performance. Since a sand blast is used on the sapphire substrate to improve uniform laser beam energy distribution, the GaN surface after laser lift-off is typically rough, which results in less light output than if it were a flat, smooth surface. In addition, the metal reflective layer formed on the n-GaN layer is not as high as non-metallic reflector material, such as ITO.
Due to these limitations of conventional techniques, a new technique is needed that can improve device performance and fabrication yield in high volume production of GaN-based semiconductor devices.